1. Field of the Invention
The present invention relates to an injection control apparatus.
2. Description of the Related Art
Conventionally, a molding machine; for example, an injection molding machine, includes a mold apparatus, a mold-clamping apparatus, and an injection apparatus. The mold apparatus includes a stationary mold and a movable mold. The mold-clamping apparatus advances and retreats the movable mold to thereby perform mold closing, mold clamping, or mold opening of the mold apparatus. As a result of mold clamping, a cavity is formed between the stationary mold and the movable mold. Meanwhile, the injection apparatus includes a heating cylinder, and a screw which is disposed in the heating cylinder such that the screw can rotate, advance, and retreat. The injection apparatus includes a metering motor, an injection motor, etc. so as to rotate and advance the screw. When the screw is rotated in a metering step, resin is accumulated forward of the screw within the heating cylinder. When the screw is advanced in an injection step, the accumulated resin is injected from an injection nozzle attached to the front end of the heating cylinder. The resin flows through a runner within the mold apparatus, and then enters the cavity via a gate, whereby the resin is charged into the cavity. The charged resin is cooled and solidified through cooling of the mold apparatus, whereby a molded product is yielded.
Incidentally, there has been proposed an injection control apparatus which controls charging of resin while switching the control mode between speed control and pressure-restricting control in order to inject the resin at high speed and under high pressure (see, for example, Japanese Patent Application Laid-Open (kokai) No. H6-55599).
FIG. 1 is a time chart showing a first charging method employed by a conventional injection apparatus; FIG. 2 is a first view showing pressure distribution within a runner as measured in the conventional injection apparatus; FIG. 3 is a first view showing pressure distribution within a cavity as measured in the conventional injection apparatus; FIG. 4 is a second view showing pressure distribution within the runner as measured in the conventional injection apparatus; FIG. 5 is a second view showing a pressure distribution within the cavity as measured in the conventional injection apparatus; and FIG. 6 is a time chart showing a second charging method employed by the conventional injection apparatus.
In the first charging method, as shown in FIG. 1, when injection is started at timing t1, velocity control is started, whereby resin fills a sprue 111 and a gate 112 of a mold apparatus (see FIG. 2) at a speed corresponding to a constant injection speed V1, with the consequence that injection pressure gradually increases. Notably, the injection speed is represented by speed of a screw, and the injection pressure is represented by force with which the screw is pushed.
Subsequently, the resin enters a gate narrow portion g at timing t2, and the resin fills the gate narrow portion g at timing t3. When the resin having passed through the gate narrow portion g enters a cavity C, the injection speed is increased sharply to V2 (V1<V2), with the consequence that the injection pressure increases sharply. When the charging of the resin into the cavity C ends at timing t4, in order to prevent generation of a sink mark, burr, etc., the injection speed is reduced to V3 (V1<V3<V2), with the consequence that the increase rate of the injection pressure decreases. Next, when the speed control is ended and pressure-restricting control is started at timing t5, the injection pressure is restricted not to exceed a restriction value Pm, with the consequence that the injection speed gradually decreases. At timing t6, VP changeover is performed, pressure-holding is started, and the control mode is switched from the pressure-restricting control to pressure control.
Incidentally, in the case where the above-described speed control is performed such that the injection speed is controlled stepwise; i.e., stepwise speed control is performed, the injection speed in the period between timings t2 and t3 may be increased to V4 (V1<V4<V3<V2) as indicated by an alternate long and short dash line in FIG. 1. In this case, the injection pressure increases as indicated by an alternate long and short dash line. As a result, as shown in FIG. 2, the pressure within the gate 112, which gradually expands toward a cavity C, becomes instable and non-uniform. In addition, when the resin fills the gate 112 at timing t3, a portion of the resin flows into the cavity C.
Further, even after the resin has entered the cavity C at timing t3, as shown in FIG. 3, the pressure within the cavity C becomes instable and non-uniform.
As a result, a molded product may involve a defect such as uneven thickness, distortion, warpage, etc., or gas may be generated in the cavity C.
In order to overcome the above-mentioned drawback, the injection speed in the period between timings t2 and t3 may be made equal to the injection speed V1 employed up to timing t2, as indicated by a broken line in FIG. 1. However, in this case, the injection speed gradually decreases in the period between timings t2 and t3, so that resin cannot be caused to smoothly fill the gate 112 as shown in FIG. 4, and a sufficient amount of resin cannot be charged into the cavity C as shown in FIG. 5.
In view of the above-mentioned problem, a second charging method as shown in FIG. 6 has been proposed. In this case, speed control and pressure-restricting control are performed during a period between timings t1 to t6; and the injection pressure is restricted such that the injection pressure does not exceed a restriction value Pm1 during a period between timings t1 to t3 and a restriction value Pm2 during a period between timings t3 to t6.
When injection is started at timing t1, velocity control is started, whereby resin fills the sprue 111 and the gate 112 at a speed corresponding to a constant injection speed V1, with the consequence that injection pressure gradually increases.
Subsequently, although the resin enters the gate 112 at timing t2, the injection pressure is restricted not to exceed the restriction value Pm1, because the pressure-restricting control is performed. In consideration of this, at timing t2, the injection speed is increased by a predetermined amount such that the injection pressure becomes the restriction value Pm1.
Next, when the resin fills the gate narrow portion g at timing t3 and the resin having passed through the gate narrow portion g enters the cavity C, the injection speed is sharply increased to V2 as described above, with the consequence that the injection pressure increases sharply.
In the conventional second charging method, since the injection pressure is restricted not to exceed the restriction value Pm1, the injection speed is gradually decreased, and, at timing t3, becomes approximately equal to the injection speed V1.
Accordingly, resin cannot be caused to smoothly fill the gate 112 as shown in FIG. 4, and a sufficient amount of resin cannot be charged into the cavity C as shown in FIG. 5. As a result, molded products involve defects such as formation of a sink mark.